1. Field of the Invention
The present invention relates to a paper coating agent and also to a heat-sensitive recording material.
2. Description of the Prior Art
It has been common practice to use polyvinyl alcohol as a clear coating agent or pigmented coating agent to improve the surface properties of paper, such as surface strength, smoothness, gloss, gas barrier properties, water resistance, printability, and solvent resistance.
There has recently been a tendency for paper to decrease in surface strength because the percentage of waste-paper and South-Sea timber in the raw material for pulp is increasing. On the other hand, the increased printing speed requires the improvement of paper in surface strength. In the case of barrier paper, higher barrier properties are required. In the case of paper used for offset printing that uses water, high water-resistant strength is required.
In order to cope with these problems, there has been proposed a paper coating agent of modified polyvinyl alcohol containing 1 to 10 mol % of .alpha.-olefin units with 4 or less carbon atoms (Japanese Patent Application Laid-open Nos. 112794/1988 and 85198/1988). However, said modified polyvinyl alcohol has a problem of low solubility in water at 90.degree. C. or more (hereinafter referred to as "boiling water"), and there has been a problem of insoluble matter in an aqueous solution of the coating agent.
The heat-sensitive recording material has come into general use as a recording medium for computers, word processors, telex, facsimiles, and calculators on account of its advantages of giving recorded images upon simple heating, requiring no complex processes for development and fixing, producing no noise at the time of recording, and giving recording systems at relatively small cost.
There is a demand for a heat-sensitive recording material capable of high-speed printing, as the printing speed of facsimiles etc. has been increasing every year. Also, there is a demand for a heat-sensitive recording material which has an excellent color developing sensitivity so that it can record images of not only characters but also drawings and photographs, accurately and neatly. Furthermore, there is a demand for a heat-sensitive recording material superior in water resistance, oil resistance, plasticizer resistance, and preservation stability.
A heat-sensitive recording materials comprises a substrate (such as paper, plastic film, or metal-deposited paper) and a heat-sensitive color developing layer applied thereon which develops a color by heat energy, on which a surface protective layer is usually applied.
The heat-sensitive color developing layer comprises a colorless or light-colored leuco dye (heat-sensitive dye), a color developing agent such as an organic acid or phenol compound which makes the leuco dye develop color upon heating, and a binder.
The binder which has been used in the heat-sensitive color developing layer is a water-soluble polymer such as polyvinyl alcohol, methyl cellulose, hydroxycellulose, polyvinylpyrrolidone, and oxidized starch. However, the heat-sensitive color developing layer obtained by the use of the conventional binder has a problem in color development sensitivity, water resistance, and preservation of images, although it is excellent in adhesion to the substrate.
The surface protective layer is made from polyvinyl alcohol, acrylamide-modified polyvinyl alcohol, acetoacetate-modified polyvinyl alcohol, or aldehydopolyacrylamide-styrene copolymer. However, the conventional surface protective layer has a problem in water resistance, oil resistance, and plasticizer resistance, which affect preservation stability.
There is sometimes applied an undercoating layer between the substrate and the heat-sensitive color developing layer, which is made from a water-soluble polymer such as polyvinyl alcohol and oxidized starch. However, the conventional undercoating layer has a problem of short life of the thermal head.
In order to cope with the above-mentioned problems, there have been proposed a heat-sensitive recording sheet comprising an overcoat layer of polyvinyl alcohol containing 1000 ppm or less of the content of sodium and a heat-sensitive recording sheet comprising a heat-sensitive color developing layer in which the binder comprises polyvinyl alcohol containing 1000 ppm or less of the content of sodium (Japanese Patent Application Laid-open No. 64330/1994). However, these conventional heat-sensitive recording sheets still have a problem in water resistance.